Liquefaction of natural gas containing nitrogen by rectification utilizing internal and external refrigeration

ABSTRACT

LIQUEFACTION OF NATURAL GAS CONTAINING NITROGEN, THE REMAINDER CONSISTING ESSENTIALLY OF LOW BOILING HYDROCARBON SUCH AS METHANE, AT LOW POWER REQUIREMENTS, BY SEPARATING THE MAJOR PORTION OF THE NITROGEN IN SUCH GAS DURING LIQUEFACTION, COMPRISING COOLING THE NATURAL GAS TO A SATURATED VAPOR, INTRODUCING THE COOLED SATURATED NATURAL GAS INTO A RECTIFICATION COLUMN, SEPARATING THE NATURAL GAS IN THE COLUMN INTO A NITROGEN OVERHEAD AND A LIQUID NATURAL GAS ENRICHED IN THE LOW BOILING HYDROCARBON, E,G., METHANE, PASSING THE OVERHEAD NITROGEN IN HEAT EXCHANGE REALTION ALONG THE COLUMN AND PASSING AN EXTERNAL REFRIGERNANT IN HEAT EXCHANGE RELATION ALONG THE COL-   UMN, FOR REMOVAL OF HEAT FROM THE COLUMN THROUGHOUT ITS LENGTH AND EFFECTING A NONADIABATIC SEPARATION OF THE NATURAL GAS IN THE COLUMN, PASSING THE RESULTING HEATED NITROGEN INTO HEAT EXCHANGE RELATION WITH THE NATURAL GAS FEED AND PASSING AN EXTERNAL REFRIGERANT IN HEAT EXCHANGE RELATION WITH THE NATURAL GAS FEED, FOR THE ABOVE-NOTED COOLING OF SUCH NATURAL GAS FEED TO A SATURATED VAPOR

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BYZUW ATTOQJ'EV United States Patent 3,559,418 LIQUEFACTION OF NATURAL GAS CONTAINING NITROGEN BY RECTIFICATION UTILIZING INTERNAL AND EXTERNAL REFRIGERATION Michael L. Hoffman, Los Angeles, Calif., assignor to McDonnell Douglas Corporation, Santa Monica, Calif a corporation of Maryland Filed Aug. 7, 1968, Ser. No. 750,996 Int. Cl. Fj 3/02 US. Cl. 62-24 15 Claims ABSTRACT OF THE DISCLOSURE Liquefaction of natural gas containing nitrogen, the remainder consisting essentially of low boiling hydrocarbon such as methane, at low power requirements, by separating the major portion of the nitrogen in such gas during liquefaction, comprising cooling the natural gas to a saturated vapor, introducing the cooled saturated natural gas into a rectification column, separating the natural gas in the column into a nitrogen overhead and a liquid natural gas enriched in the low boiling hydrocarbon, e.g., methane, passing the overhead nitrogen in heat exchange relation along the column and passing an external refrigerant in heat exchange relation along the column, for removal of heat from the column throughout its length and effecting a nonadiabatic separation of the natural gas in the column, passing the resulting heated nitrogen into heat exchange relation with the natural gas feed and passing an external refrigerant in heat exchange relation with the natural gas feed, for the above-noted cooling of such natural gas feed to a saturated vapor.

This invention relates to the liquefaction of natural gas containing nitrogen and low boiling hydrocarbon such as methane, to substantially reduce the power required for liquefaction, and is particularly concerned with procedure for carrying out such liquefaction, by means of gas separation techniques, in which the nitrogen present in the natural gas is separated therefrom during the liquefaction process, employing low temperature nonadiabatic distillation and external refrigeration to provide efficient liquefaction and nitrogen separation.

Natural formations of natural gas often contain significant quantities of nitrogen. While it is not usually necessary to remove nitrogen for the commercial use of the natural gas as a fuel, the presence of nitrogen substantially increases the cost and difiiculty of liquefying natural gas. There are two effects of nitrogen on the liquefaction process. One effect is having to condense substantially all the nitrogen in order to recover as liquid all the natural gas in the feed. The other effect is the general lowering of the average condensing temperature of all the fluid due to the low boiling point temperature 'of the nitrogen compared to other constituents of natural gas.

The significance of the effect of nitrogen present in natural gas on the additional work required for liquefaction may be appreciated by calculating the theoretical minimum work of liquefaction for pure methane and for a nitrogen-70% methane mixture. In the case of "pure methane at 100 p.s.i.a. the theoretical minimum work of liquefaction is approximately 6100 B.t.u./mole. The work required to liquefy a mixture of 70% methane and 30% nitrogen is approximately 6680 B.t.u./mole of mixture, or 9540 B.t.u./mole of methane.

According to the invention, separation of the nitrogen is employed as an essential feature to obtain efficient liquefaction. Thus, it is possible to separate the nitrogen during liquefaction of the natural gas, and thus reduce the 3,559,418 Patented Feb. 2, 1971 ice amount of work required to obtain the desired quantity of liquid methane. Thus, taking for example the abovenoted 30-70% methane natural gas mixture, the theoretical work of separation of this mixture would be 6370 B.t.u./mole of mixture, or 9100 B.t.u./mole of methane product. Thus, by separating the nitrogen from the methane the total minimum theoretical work requirement is reduced from 9540 B.t.u./mole of methane to 7010 B.t.u./ mole of methane.

An additional advantage of the invention process is that the liquefied natural gas product having a substantially lower nitrogen content and a substantially higher methane content, has a substantially increased B.t.u. content per unit of product, and thus has greater fuel value.

The separation of the major portion of the nitrogen content of the natural gas is effected during liquefaction of the natural gas by low temperature rectification. In accordance with the invention, this separation is carried out nonadiabatically, that is, heat is removed from the column throughout the length of the distillation zone, thus allowing the required refrigeration to be provided at temperature levels approaching the maximum allowable for that point in the process. This results in a reduced work requirement for production of the required refrigeration.

More specifically, the invention provides a process for liquefying natural gas containing from about 5% to about 60% nitrogen, the remainder consisting essentially of low boiling hydrocarbon, usually chiefly methane, which comprises cooling the natural gas to a saturated vapor, introducing the cooled saturated natural gas as feed into a rectification column, effecting a separation of the natural gas in the column, withdrawing overhead nitrogen from the upper end of the rectification column, withdrawing liquid natural gas product substantially enriched in the low boiling hydrocarbon and substantially reduced in nitrogen content, from the lower end of the rectification column, passing the overhead nitrogen in heat exchange relation with the rectification column substantially throughout the length thereof, and passing an external refrigerant in heat exchange relation with the rectification column substantially throughout the length thereof, to remove heat from the column throughout its length, and to effect a nonadiabatic separation of the natural gas in the column.

In preferred practice, heated nitrogen exiting from heat exchange relation with the rectification column, is passed into heat exchange relation with the natural gas feed, and an external refrigerant is also passed in heat exchange relation with the natural gas feed, to provide the above-noted cooling of the natural gas to a saturated vapor.

The natural gas feed, as noted above, generally contains from about 5% to about 60% nitrogen, the remainder being essentially low boiling hydrocarbon, par ticularly methane, and also generally containing minor amounts of other low boiling hydrocarbons such as ethane and propane. The natural gas feed can be provided at relatively low pressure, e.g., ambient pressure, or at high pressures, e.g., up to 400 p.s.i.a., or higher. Various modifications of the process are practiced, as will be pointed out more fully hereinafter, depending upon the pressure of the natural gas feed.

Where the natural gas feed is at relatively elevated pressures, e.g., between about 50 and about 400 p.s.i.a., the cold overhead nitrogen from the rectification column, after passage inheat exchange relation along the length of the column, as previously noted, can be work expanded. to substantially reduce its temperature, and the cold work-expanded nitrogen again passed in heat exchange relation along the rectification column substantially throughout the length thereof, for providing further 3. refrigeration, and the heated exiting nitrogen then passed in heat exchange relation with the natural gas feed to aid in cooling the same.

Where the natural gas feed is at relatively elevated pressures and the rectification column is also operated at elevated pressure, the hydrocarbon-enriched liquid bottoms product can be subcooled against an external refrigerant stream and throttled to a desired or predetermined storage pressure. Alternatively, such hydrocarbonenriched liquid product can be throttled to storage pressure and the flash vapor passed in heat exchange relation with the natural gas feed to aid in cooling same, the exiting throttled hydrocarbon vapor compressed and recycled by admixture thereof with the natural gas feed.

Where the feed is at a substantially higher pressure than the operating pressure in the column, following initial cooling of the natural feed gas mixture by countercurrent heat exchange with cold. overhead nitrogen and external refrigerant, such high pressure feed can be work expanded to the column operating pressure, thus further cooling the natural gas feed to a saturated vapor for introduction into the rectification column, and thereby reducing the amount of external refrigeration required in said initial cooling stage.

The invention will be understood more clearly by the description below of certain embodiments of the invention taken in connection with the accompanying drawing wherein:

FIG. 1 is a schematic representation illustrating the basic process and system of the invention, designed particularly for operation with a low pressure natural gas feed;

FIG. 2 is a schematic representation of a modified form of the invention, employing a relatively high pressure natural gas feed, With the liquid natural gas product being subcooled and throttled. to storage temperature;

FIG. 3 is a schematic representation of another modification of the invention process employing a high pressure natural gas feed, with the enriched natural gas liquid product throttled to storage pressure and the flash vapor recycled;

FIGS. 4 and 5 are schematic representations of modifications of the processes of FIGS. 2 and 3, respectively, employing a natural gas feed at a pressure higher than the operating pressure in the rectification column.

In the various figures of the drawing, the same reference numerals represent the same components.

Referring to FIG. 1, the natural gas feed mixture containing 70% methane and 30% nitrogen, and at low pressure ranging from about 15 to about 50 p.s.i.a., specifically 15 p.s.i.a. in the present example, and at a temperature of 20 C., is introduced at into a precooler 12, and passed through coil 14 thereof in countercurrent heat exchange relation with an external refrigerant 15 such as evaporating propane liquid, passed through coil 16 of the precooler, and with cold waste nitrogen passing through coil 18 of the precooler.

The exiting cooled feed leaving the precooler at 20 as saturated vapor at 162 C., is fed to the bottom of a rectification column 22 of the separation unit 23. As the vapor travels up the column 22, it is partially condensed by an external refrigerant stream 24, such as liquified methane or ethylene, passing through the passages 26 in countercurrent heat exchange relation with column 22 throughout the length of the column. The vapor in column 22 thus becomes leaner in hydrocarbon, i.e., methane, and richer in nitrogen. A liquid product nearly in equilibrium with the feed vapor and thus substantially enriched in such hydrocarbon, is withdrawn at 28 from the bottom of the column, while a colder nitrogen vapor containing substantially no hydrocarbon, i.e., methane, is withdrawn at 30 from the top of the column. Such overhead nitrogen vapor at a temperature of about 192" C., is then warmed by passage through the heat exchange passages 32 throughout the le gth of the column, and withdrawing heat from the column to augment the refrigeration required in the column.

The heated exiting nitrogen at 34 then passed through coil 18 of the precooler 12, to provide additional refrigeration therein, and the warm nitrogen waste stream at approximately ambient temperature is then discharged from the precooler at 36. In the present example, where the column 22 is operated at 15 p.s.i.a. and assuming the feed contains 70% methane and 30% nitrogen, the enriched methane liquid product at 28 would contain only about 2% nitrogen.

From the description above of the process illustrated in FIG. 1, it is seen that the liquefaction of a natural gas feed containing 30% nitrogen and the remainder low boiling hydrocarbon, is readily carried out by separation of the nitrogen in a rectification column during liquefaction of the natural gas feed, such separation being efliciently carried out by a nonadiabatic rectification employing an external refrigerant together with cold overhead nitrogen passing substantially throughout the length of the column to supply the requisite refrigeration, and providing an enriched liquid hydrocarbon or liquid natural gas product containing only about 2% nitrogen.

Referring now to FIG. 2 of the drawing, there is illustrated a. modification of the invention process for liquefying the natural gas feed at moderately elevated pressures, for example between about 50 and about 400 p.s.i.a. In this modification, assuming a natural gas feed containing 30% nitrogen and 70% methane, at a feed pressure of p.s.i.a., the feed gas is cooled following passage through precooler 12, to a saturated vapor at 20, at a temperature of about C. The cooled feed at 20 is fed to the bottom of the separation column 22, operating at a pressure of about 100 p.s.i.a., where it is separated into a bottoms liquid at 28 containing 95% methane and 5% nitrogen, and a substantially pure nitrogen overhead vapor is withdrawn at 30 at a temperature of about l75 C.

Following passage of the nitrogen vapor through the heat exchange passages 32 in heat exchange relation with column 22, the Warm nitrogen vapor at 34 is then introduced at 38 into an expander or turbine 40 where the nitrogen vapor discharged at 42 is expanded to near atmospheric pressure of about 20 p.s.i., reducing the temperature of the nitrogen to C. The discharge 42 from expander 40 is then warmed by passage through heat exchange passages 44 of the separation column 22, extending throughout the length of the column, the exiting heated nitrogen leaving at 46 being then fed through coil 18 of precooler 12 and further warmed therein to near ambient temperature upon discharge at 36.

As in the case of the process and system of FIG. 1, additional refrigeration required in column 22 is provided by external refrigerant stream 24 passing through passages 26 in heat exchange relation along column '22. The bottoms liquid product at 28 is then subcooled by passage through coil 48 of subcooler 50, against an external refrigerant 52, such as liquefied ethylene, passed through coil 54 of subcooler 50, the enriched hydrocarbon or methane product being thus sufficiently subcooled so that it can be throttled at 56 to storage pressure, e.g., of about 30 p.s.i.a., at 58, without appreciable flash.

The process and system illustrated in FIG. 3 is similar to that described above and illustrated in FIG. 2, except that the bottoms liquid product at 28 is not subcooled with external refrigerant. Instead, the liquid hydrocarbon product stream 28 is partially subcooled in subcooler 60 by passage through coil 62 therein, against return hydrocarbon product flash vapor passing through coil 64. The partially subcooled stream is throttled at 66 to storage pressure of about 30 p.s.i.a., and the throttled stream is then introduced into a liquid-vapor separator 63.

The liquid product formed in separator 68 is withdrawn at 70 from the bottom of the separator, and flash vapor from the top is withdrawn at 72 and is warmed first by passage at 64 through the subcooler 60, and is further warmed by passage through coil 74 of the precooler 12, from which it is discharged at 76, at near ambient temperature. The exiting heated methane vapor at 76 is then compressed at 78 to feed gas pressure of 100 p.s.i.a., the compressed methane then being cooled in cooler 80 with water, and recycled and mixed at 82 with the natural gas feed 10.

In the processes and systems of FIGS. 4 and 5, which are modifications of FIGS. 2 and 3, respectively, the natural gas feed is at a pressure above What would be considered desirable operating pressure in the column. Thus, for example, the pressure of the natural gas feed at 10, in FIGS. 4 and 5, could be above 400 p.s.i.a., e.g., 600 p.s.i.a. In order to utilize the energy of this highpressure feed, the feed 10, e.g., a natural gas feed containing 30% nitrogen and 70% methane, is cooled to a temperature in precooler 12, of about -100 C., which is above the saturated vapor temperature. This reduces the amount of refrigeration required to be supplied by the external refrigerant at 15, passing through coil 16 of the precooler. The cooled compressed feed at 20 is then expanded in the expander or turbine 84 to column operating pressure, e.g., about 400 p.s.i.a., and saturated vapor temperature of about ll0 C., and the expanded saturated vapor at 86 is then introduced into the rectifying column 22, and the remainder of the processes illustrated in FIGS. 4 and 5 are carried out the same as described above with respect to FIGS. 2 and 3, respectivel In all of the processes described above and illustrated in FIGS. 1 to 5, refrigeration is to be supplied at the temperature levels required. Thus, refrigerants 15, 24 and 52 can be fluids with finite specific heats, or can be evaporating fluids at several discrete pressure levels. As examples of refrigerant fluids with finite specific heats, there are included vapors cooled by expansion such as work expanded nitrogen, methane vapor, or air. As examples of evaporating fluids there can be employed liquid methane, ethylene, ethane, propane, and the like, employed as evaporating fluids to provide refrigeration. This permits the process of the invention to be operated in a manner more closely approximating reversibility. If all of the column refrigeration were provided by condensing overhead nitrogen vapor as in conventional distillation, all of the refrigeration, e.g., as supplied at 26 according to the invention, would have to be supplied at the coldest temperature in the system, thus substantially increasing the work of refrigeration.

It will be understood that the processes and systems described above, including the temperatures and pressures set forth, are only illustrative and are not intended as limitative of the invention.

The heat exchange passages 0r constructions 26 for the refrigerant 24, and 32 and 44 for the overhead nitrogen, and the associated liquid-vapor passages 22 of the rectification column 22, all contained within the rectification units 23, can be in the form of a unitary plate-fin heat exchanger (not shown), wherein the passages or channels 26 for the refrigerant 24, and the passages or channels 32 and 44 for the nitrogen also employed for refrigeration, are arranged in indirect heat-exchange relation with the passages or channels 22, bearing the liquid-vapor natural gas mixture being separated in column 22. The channels 22' in the rectification column can be constructed in the manner of a series of perforated fins, illustrated at-22",

producing the effect of distillation column trays. This is a 6 tion column 22 and its associated refrigerant and nitrogen passages 26 and 32, 44 in indirect heat exchange relation with each other, as described above and shown in FIGS. 1 to 5, so as to effect the above-described nonadiabatic separation or rectification in the rectifying coluumn 22.

It is particularly noted that an important feature of the invention process and system is liquefaction of a natural gas containing nitrogen, and that the above-described nonadiabatic separation is an essential feature of the invention required to obtain efficient liquefaction. Under these circumstances, it is not essential that a pure hydrocarbon product be produced, but rather that as eflicient liquefaction as possible be obtained for a natural gas feed containing a given amount of nitrogen and at a particular pressure, regardless of the actual amount of nitrogen present in the liquefied hydrocarbon product. Thus, according to the invention process, such liquefied product can contain, for example, from about 1 to about 15% nitrogen, depending upon the amount of nitrogen in the feed gas, its pressure and other variables of the particular separation procedure employed. The result is a substantially reduced power requirement for liquefying the natural gas feed containing nitrogen, as compared to prior art processes, the ability to liquefy such a natural gas economically being of particular importance and the dominating economic factor as to the feasibility of shipping and employing such natural gas at localities remote from its source.

While I have described particular embodiments of my invention for the purpose of illustration, it should be understood that various additional modifications and adaptations thereof may be made within the spirit of the invention, and within the scope of the appended claims.

I claim:

1. The process for liquefying natural gas containing from about 5% to about 60% nitrogen, the remainder consisting essentially of low boiling hydrocarbon, which comprises cooling said natural gas to a saturated vapor, introducing said cooled saturated natural gas as feed into a rectification column, effecting a separation of said natural gas in said column, withdrawing overhead nitrogen from the upper end of said rectification column, withdrawing liquid natural gas product substantially enriched in said low boiling hydrocarbon and substantially reduced in nitrogen content, from the lower end of said rectification column, passing said overhead nitrogen in heat exchange relation with said rectification column throughout the length thereof and passing an external refrigerant from an external source in heat exchange relation with said rectification column, throughout the length theerof, to remove heat from said column throughout its length, and to effect a nonadiabatic separation of said natural gas in said column.

2. A process as defined in claim 1, including passing heated nitrogen exiting from heat exchange relation With said rectification column, into heat exchange relation with said natural gas feed, and passing an external refrigerant in heat exchange relation with said natural gas feed, for cooling said natural gas to said saturated vapor.

3. A process as defined in claim 1, wherein said low boiling hydrocarbon is methane.

4. A process as defined in claim 1, wherein said natural gas feed is at elevated pressure and said overhead nitrogen is at elevated pressure, and following passage of said overhead nitrogen in heat exchange relation with said rectification column, said nitrogen is work expanded to substantially reduce its temperature, and the cold work expanded nitrogen again passed in heat exchange relation along said rectification column throughout its length, for providing further refrigeration.

5. A process as defined in claim 1, wherein said natural gas feed is at elevated pressure, and said liquid natural gas product is at elevated pressure, and including passing said liquid natural gas into heat exchange relation with an external refrigerant to subcool said liquid natural gas 7 product, and throttling said subcooled natural gas product to a predetermined storage pressure.

6. A process as defined in claim 1, wherein said natural gas feed is at elevated pressure, and said liquid natural gas product is at elevated pressure, and including subcooling said liquid natural gas product, throttling said subcooled natural gas product to a predetermined pressure, withdrawing throttled liquid natural gas product of reduced nitrogen content, and including passing the flash hydrocarbon vapor following throttling of said natural gas product, into heat exchange relation with said liquid natural gas product prior to throttling for said subcooling thereof, passing the exiting heated flash vapor into heat exchange relation with said natural gas feed for cooling same, compressing the resulting heated flash hydrocarbon vapor approximately to the elevated pressure of said natural gas feed, and recycling said compressed hydrocarbon vapor by admixture thereof with said natural gas feed.

7. A process as defined in claim 1, wherein said natural gas feed is at an elevated pressure substantially higher than the operating pressure in said column, and including passing said heated nitrogen exiting from heat exchange relation with said rectification column, into heat exchange relation with said natural gas feed and passing an external refrigerant in heat exchange relation with said natural gas feed, for initially cooling said compressed natural gas feed, and work expanding said initially cooled compressed natural gas feed to said column operating pressure and to a reduced temperature providing a saturated vapor, followed by said introduction of said cooled saturated natural gas feed into said rectification column.

8. A process as defined in claim 1, wherein said low boiling hydrocarbon is methane, and wherein said liquid methane product contains a substantially reduced nitrogen content of between about 1 and about 15% nitrogen.

9. A process as defined in claim 2, wherein said natural gas feed is at elevated pressure and said overhead nitrogen is at elevated pressure, and following passage of said overhead nitrogen in heat exchange relation with said rectification column, said nitrogen is work expanded to substantially reduce its temperature, and the cold work expanded nitrogen again passed in heat exchange relation along said rectification column throughout its length, for providing further refrigeration, prior to passage of the heated nitrogen into heat exchange relation with said natural gas feed for cooling same.

10. A process as defined in claim 9, wherein said low boiling hydrocarbon is methane, and wherein said liquid methane product contains a substantially reduced nitrogen content of between about 1 and about 15 nitrogen.

11. A process as defined in claim 9, wherein said liquid natural gas product is at elevated pressure, and including passing said liquid natural gas into heat exchange relation with an external refrigerant to subcool said liquid natural gas product, and throttling said subcooled natural gas product to a predetermined storage pressure.

12. A process as defined in claim 11, wherein said natural gas feed is at elevated pressure, and said liquid natural gas product is at elevated pressure, and including subcooling said liquid natural gas product, throttling said subcooled natural gas product to a predetermined pressure, withdrawing throttled liquid natural gas product of reduced nitrogen content, and including passing the flash hydrocarbon vapor following throttling of said natural gas product, into heat exchange relation with said liquid natural gas product prior to throttling for said subcooling thereof, passing the exiting heated flash vapor into heat exchange relation with said natural gas feed for cooling same, compressing the resulting heated flash hydrocarbon vapor approximately to the elevated pressure of said natural gas feed, and recycling said compressed hydrocarbon vapor by admixture thereof with said natural gas feed.

13. A system for liquefying natural gas containing from about 5% to about 60% nitrogen, the remainder consisting essentially of low boiling hydrocarbon, which comprises means for cooling said natural gas to a saturated vapor, a rectification column, means for introducing said cooled saturated natural gas as feed into said rectification column, to effect a separation of said natural gas in said column, means for withdrawing overhead nitrogen from the upper end of said rectification column, means for withdrawing liquid natural gas product substantially enriched in said low boiling hydrocarbon and substantially reduced in nitrogen content, from the lower end of said rectification column, means for passing said overhead nitrogen in heat exchange relation with said rectification column throughout the length thereof and means for passing an external refrigerant from an external source in heat exchange relation with said rectification column, throughout the length thereof, to remove heat from said column throughout its length, and to effect a nonadiabatic separation of said natural gas in said column.

14. A system as defined in claim 13, including means for passing heated nitrogen exiting from heat exchange relation with said rectification column, into heat exchange relation with said natural gas feed, and means for passing an external refrigerant in heat exchange relation with said natural gas feed, for cooling said natural gas to said saturated vapor.

15. The process for liquefying natural gas containing from about 5% to about 60% nitrogen, the remainder consisting essentially of low boiling hydrocarbon, which comprises providing a natural gas feed at an elevated pressure, cooling said natural gas feed, work expanding said cooled compressed natural gas feed to a saturated vapor, introducing said cooled saturated natural gas as feed into a rectification column, said initial natural gas feed being at an elevated pressure substantially higher than the operating pressure in said column, said natural gas feed being work expanded to column operating pressure, effecting a separation of said natural gas in said column, withdrawing overhead nitrogen at elevated pressure from the upper end of said rectification column, withdrawing liquid natural gas product at elevated pressure substantially enriched in said low boiling hydrocarbon and substantially reduced in nitrogen content, from the lower end of said rectification column, passing said overhead nitrogen in heat exchange relation with said rectification column throughout the length thereof, work expanding said nitrogen exiting from heat exchange relation with said rectification column, to substantially reduce the temperature of said exiting nitrogen, again passing the cold work expanded nitrogen in heat exchange relation along said rectification column throughout its length, for providing further refrigeration, passing an external refrigerant in heat exchange relation with said rectification column throughout the length thereof, to remove heat from said column throughout its length, and to effect a nonadiabatic separation of said natural gas in said column, passing the heated nitrogen exiting from heat exchange relation with said rectification column, into heat exchange relation with said natural gas feed, passing an external refrigerant in heat exchange relation with said natural gas feed, for said cooling said natural gas feed prior to work expansion thereof, subcooling said liquid natural gas product, throttling said subcooled natural gas product to a predetermined pressure, withdrawing throttled liquid natural gas product of reduced nitrogen content, passing the flash hydrocarbon vapor following throttling of said natural gas product, into heat exchange relation with said liquid natural gas product prior to throttling for said subcooling thereof, passing the exiting heated flash vapor into heat exchange relation with said natural gas feed for cooling same, compressing the resulting heated flash hydrocarbon vapor approximately to the elevated pressure of said natural gas feed, and recycling said compressed hydrocarbon vapor by admixture thereof with said natural gas feed.

(References on following page) 9 References Cited UNITED STATES PATENTS 6/1934 D6 Baufre 6231 3 407 14 10 19 6/1938 Pollitzer 62--39 4/1950 Haynes 6229 5/1951 Patterson 6231 5/1954 Miller 6231 7/1955 Williams 6231 62 31., 39, 40

10 Eakin et a1 62--39 Greenberg et a1. 6229 Jakob 62-39 Poska 6240 5 WILBUR L. BASCOMB, JR., Primary Examiner 1 US. Cl. X.R. 

